A metal detection apparatus is used to detect metal contamination in edible goods and other products. As described in WO02/25318, modern metal apparatuses utilise a search head comprising a “balanced coil system” that is capable of detecting all metal contaminant types including ferrous, nonferrous and stainless steels in a large variety of products such as fresh and frozen products.
As described in U.S. Pat. No. 8,841,903 B2, a metal detection apparatus that operates according to the “balanced coil”-principle typically comprises three coils that are wound onto a non-metallic frame, each coil exactly parallel with the other. The transmitter coil located in the centre is energised with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed with the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the first receiver coil is connected in series with the second receiver coil having an inversed sense of winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity are cancelling out one another in the event that the system, in the absence of metal contamination, is in balance.
As a metal object passes through the coil arrangement, the high frequency field is disturbed first near the first receiver coil and then near the second receiver coil. While the metal object is conveyed through the receiver coils the voltage induced in each receiver coil is changed typically in the range of nano-volts. This change in balance results in a signal at the output of the receiver coils that can be processed, amplified and subsequently be used to detect the presence of metal contamination in a product.
The signal processing channels normally split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal object, “product effects” need to be removed or reduced. If the phase of the product is known then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the contaminants, the product and other disturbances have different influences on the magnetic field so that the resulting signals differ in phase.
The signals caused by various metals or products, as they pass through the coils of the metal detection apparatus, can be split into two components, namely resistive and reactive components, according to conductivity and magnetic permeability of the measured object. The signal caused by ferrite is primarily reactive, while the signal caused by stainless steel is primarily resistive. Products, which are conductive typically cause signals with a strong resistive component.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants.
In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
With the arrangement disclosed in U.S. Pat. No. 8,841,903 B2, the resonant circuit, which consists of the transmitter coil and one or more tuning capacitors, can be tuned optimally and independently of other parts of the transmitter unit to the selected transmitter frequency.
This arrangement however does not address the problem that metal contamination in the product does often not provide sufficient signal response. Non-spherical metal objects will provide a signal response, which depends on the consistency of the material and its orientation to the incident magnetic field. In the metal detection system disclosed in U.S. Pat. No. 8,841,903 B2 the coils are placed in the yz-plane and the magnetic field extends along the x-axis. In unfavourable orientations, non-spherical metal may therefore not cause detectable field changes.
Because of their properties, ferrous and non-ferrous metals interact differently with the magnetic field. Ferrous metals have a magnetic permeability higher than air and therefore attract the field. When a ferrous wire is placed with a short edge leading in a metal detector, the wire is aligned in parallel to the magnetic field and causes a maximum field disturbance compared to the same wire orientated perpendicular thereto. With the first orientation of the wire magnetic flux lines are attracted and can extend over a longer distance within the wire, which therefore has a strong impact on the magnetic field. With the second orientation of the wire, magnetic flux lines only traverse the wire along a very short distance thus having a small impact on the magnetic field.
Non-ferrous metals, such as stainless steel, copper, aluminium, brass, have magnetic permeability similar to air. Therefore, the metal detector is not detecting these metals because of the change in permeability, but due to the occurrence of an alternative magnetic field caused by eddy currents that are created in the non-ferrous wire. The induced eddy currents create a magnetic field which opposes the field generated by the transmitter and reduces it locally. When a non-ferrous wire is placed with a long edge leading in a metal detector the cross-section of the wire exposed to the flow of the field is larger generating stronger eddy currents which create a stronger opposing field. In this orientation of the wire the disturbance is greater compared to orientation of this wire introduced with the short edge leading.
Consequently, depending on their consistency and orientation a metal detection apparatus may be able to detect such metal objects or not.
In order to detect metal objects with a specific orientation product could be transferred through different metal detection apparatuses which are placed one behind the other with different orientation. Such a setup of course involves considerable costs and efforts by the operator.
In an article titled “Two Channel Metal Detector using Two Perpendicular Antennas”, Kyoo Nam Choi, of Incheon National University, 2013, describes a two-channel metal detection apparatus having two sets of perpendicularly oriented sensor antennas, which allow expanding the detectable size of metal particles. It is stated that a single channel metal detection sensor has not shown sensitivity resolution through a wide range of metal size. Thus, there was a need to cascade the sensors having different sensitivity resolutions.
Instead of arranging different metal detection systems, e.g. with different angle of placement, Kyoo Nam Choi proposes to use two independent sensor channels each provided with a set of antennas arranged perpendicular to one another. This arrangement requires a complex arrangement and considerable circuitry to process and evaluate the signals gained by the two antenna systems.
Japanese application JPS 57127868 A discloses a system with a plurality of rectangular excitation coils, with which magnetic fields can be applied to the product and contaminant from various directions. While the plurality of magnetic fields may enhance detectability of metal contaminations with specific form and orientation, processing the resulting signals, which will comprise various signal components with different phase and amplitude, will be difficult. Signal components of the product or contaminants may add up or cancel out possibly causing false-positive and false-negative reports of the metal detection system. Furthermore, installing and operating a plurality of excitation coils requires space and an enhanced transmitter system. Still further, mounting different excitation coils typically requires a larger volume of the coil system, thus reducing the coupling of the current system to the contaminants so that sensitivity is reduced.
It is further important to note that known metal detection apparatuses may incorporate coil systems that define detection zones having different geometrical forms. The metal detection apparatus and the geometrical form of the detection zone, which forms the passage channel for the processed products, are selected according to the application process. Often the detection zone or passage channel has a rectangular cross-section through which products are transferred by a conveyor belt. Hollow cylindrical or conical detection zones are often used in metal detection apparatuses that are used in processes, in which a product is vertically dropped into a container. Hence, the detection zones may have a cross-sectional profile that varies or is constant along the travel path of the product. Systems with conical detection zones use coils that differ in size from one another typically with the transmitter coil being off-centred between the two receiver coils. In both systems, the coils are arranged such that, when the at least one transmitter coil is energized by an alternating electric current, the electromagnetic field generated thereby induces a first voltage in the first receiver coils and a second voltage in the second receiver coil, the first and second voltages cancelling each other out when there is no metal present in the object under inspection.
The present invention is therefore based on the object of creating an improved metal detection apparatus.
In particular, the present invention is based on the object of creating an improved metal detection apparatus that allows reliable detection of metal particles independently of their consistency, geometrical form and orientation.
Particularly, the present invention is based on the object of creating a metal detection apparatus that operates with improved signal sensitivity for metal particles independently of their consistency, geometrical form and orientation.
The metal detection system shall provide improved results without requiring additional coil systems, processing channels, or additional processing efforts. Further, the metal detection apparatus shall still have a compact structure and practically equivalent dimensions compared to known metal detection apparatuses.
Furthermore, the metal detection system shall be created such that a close coupling of the coil system to contaminants contained in a processed product is reached.
Still further, the invention shall be applicable to any type of metal detection apparatus with any kind of detection zone or passage channel that may be for example rectangular, hollow cylindrical or conical.